Manufacturers of polyphase motors specify the range of operating voltages and the amount of voltage unbalance that may be tolerated by the motors. Voltage unbalance, a condition in which the various phase voltages are unequal, causes wasteful heating in the rotor windings of a conventional squirrel cage motor. This can occur even though all of the phase voltages are within generally accepted voltage limits.
Various circuits have been proposed and are used to monitor multiphase power lines. Some of these, using multiphase full wave rectifiers, are responsive to changes in the ripple voltage of the rectifiers. Others compare part, usually half, of the line to line voltage of one phase with a phase shifted portion of another line to line voltage. These circuits, both rectifier ripple voltage and phase shift circuits are quite satisfactory if the voltage wave forms are pure sine waves. They are not satisfactory if the voltages being monitored are distorted, i.e. include harmonics of the fundamental voltage. The harmonics cause tripping on types of unbalance which do no harm to the motors. Monitors responsive to the ripple voltage of rectifiers connected to the lines being monitored are also responsive to or affected by the type of unbalance, thus tripping at differing amounts of unbalance depending upon the type of unbalance occurring.